Electric lamps are efficient converters of electric power to heat energy. The energy is emitted as conduction-convection energy and as radiant energy. The latter includes infrared (IR), visible emission and ultraviolet (UV).
Good building design seeks to provide efficient utilization or dissipation of the lighting heat. Heat transfer luminaires for fluorescent lamps as well as heat transfer luminaires for incandescent lamps have been employed. Both air and water have been used as control mechanisms for the removal of lighting heat. The potential for heat transfer with high intensity discharge lamps in several prototype luminaires is described in the paper "Heat Transfer With High-Intensity Discharge Lamps and Luminaires" by W. S. Fisher and S. Weinstein (Illuminating Engineering, Vol. 65, April, 1970, Page 185).
In most of the instances in which air was employed as the medium for removing lighting heat and this air will have been deliberately or inherently moved over the surface of the lamp, primary reliance appears to have been placed upon contact between the air flow and the luminaire. In contrast thereto the instant invention focuses directly upon the lamp body for the recovery of heat therefrom as will be described hereinafter.
An application of prime interest for this invention is in the field of controlled environment agriculture, wherein high intensity discharge lamps are employed to provide all or part of the photosynthetic light required for the growth of plants. Thus, the thermal radiation emanating from a high intensity discharge lamp presents two major problems. In addition to the desired photosynthetic wavelengths, radiant thermal energy is also reflected downward by the lamp reflector onto the leaf surfaces. Since this additional thermal load can result in excess heating of the leaf, the thermal loading so imposed will often limit the intensity at which lamps in such establishments can be operated. A reduction in the thermal radiation reaching the plants at any given operating intensity is highly desirable, since this will permit an increase in the light intensity utilized, which in turn increases the intensity of the photosynthetic light to which the plants can be exposed with consequent advantages in growth rates. Also, a more general problem arises from the need to maintain a constant temperature within the controlled environment structure. The non-visible energy output from the lamp imposes a penalty (i.e., introduces an additional cooling load) without serving any useful purpose. It is, therefore, desirable to reduce the transfer of heat from the high intensity discharge lamps to the ambient (i.e., lamp surroundings) so that the cooling load handling capacity of the air conditioning can be reduced. At present, this high cooling duty is apparently a major problem even in colder climates, cooling being required during the period when the lights are operating and heating being required when the lights are turned off. The same basic need for reducing the air conditioning load imposed by lighting heat applies in industrial establishments, etc.
Thus, this art is in need of a relatively inexpensive lamp construction adapted to provide the capability for extracting as large a fraction of non-visible, thermal energy from high intensity discharge lamps at as high a temperature level as possible without significantly reducing the visible radiation.